A steam reforming process utilizing catalysts, which produce a mixture of hydrogen and carbon monoxide, so called a synthesis gas, by using steam as an oxidant for methane which is a major component of natural gas has been already developed and industrialized. The synthesis gas made by the steam reforming of methane is applicable to Fischer-Tropsch synthesis which is a process for synthesizing ammonia and hydrocarbon, and to a process for producing oxygen-containing compound like methanol. However, this steam reforming process to give a synthesis gas has problems that the molar ratio of hydrogen to carbon monoxide in obtained synthesis gas mixture is restricted to 3:1 or more. Thus it is difficult to apply the process to raw material for oxygen-containing compound and Fischer-Tropsch synthesis due to the small content of carbon monoxide. Therefore, a synthesis gas having high content of carbon monoxide which can be applicable variously not only to Fischer-Tropsch synthesis but also to hydroformylation and carbonylation is highly demanded. Another problem of steam reforming is the deactivation of the catalyst due to coke formation from methane at high temperature. In particular, it is known that deactivation of catalyst due to coke formation is severe in case of nickel catalyst which is representative steam reforming catalyst. Accordingly, in order to inhibit the coke formation owing to methane, steam and methane as reactant gases have been used in a molar ratio of 4:1 or more in practical processes, but an excessive steam can also cause deactivation of catalyst since it accelerates sintering of metals which are active component of the catalyst.
Recently, technologies to control the amount of carbon dioxide to meet a worldwide movement to control total amount of carbon dioxide have been studied in many ways in order to prevent the global warming owing to green house effect wherein carbon dioxide is pointed out as its main suspicious component.
In case of producing a synthesis gas by replacing steam with carbon dioxide in the reforming reaction of methane, a ratio of hydrogen to carbon monoxide in the reforming reaction using carbon dioxide is low as compared with that of the steam reforming reaction and this ratio can be controlled within wide range from 0.5:1 to 2:1 when a molar ratio of carbon dioxide to methane is changed. However, this process can cause catalyst deactivation due to severe coke formation on catalyst surface as in the steam reforming.
To alleviate the problem of coke formation, U.S. Pat. No. 5,068,057 suggested use of noble metal catalysts. In the specification, it was reported that in case of using commercially available Ni catalyst as steam reforming catalyst in converting carbon dioxide into carbon monoxide, the conversion reaction was ceased in less than two hours because the blockage of reactor tube was occurred due to both accumulation of coke and degradation of catalyst structure. Moreover, Pt/alumina and Pd/alumina catalysts were suggested as active catalysts in the conversion reaction of carbon dioxide into carbon monoxide using hydrocarbon.
International laying-open WO 92 11,199 (1992) disclosed that alumina supported noble metal catalysts such as iridium, rhodium and ruthenium had high activity and long life. But the above noble metal catalysts had a problem of high price.